1. Field of the Invention
The present invention relates to a sigma-delta modulator for conversion of a an analog or digital low-frequency input signal of a high resolution into a quantized analog or digital signal, which has an error feedback circuit for suppression of quantization errors. It also relates to a method for suppressing quantization errors in this type of sigma-delta modulator.
2. Prior Art
Sigma-delta modulators convert low frequency signals of high resolution, which can be in analog or digital form, into scanned, coarse quantized signals with a comparatively high scan rate. This output signal can similarly be prepared in analog or digital form. Quantization noise arises because of this coarse quantization (often only a single bit is used for the quantization). An attempt is made to shape this noise by error feedback so that the noise spectrum in the interesting low frequency band (the frequency band for the input signal) is very small, but increases at higher frequencies. High frequency noise may be largely eliminated by a low-pass filter.
A sigma-delta modulator can be used, for example, as an analog/digital converter (A/D converter); an analog input signal is converted into a high frequency digital pulse sequence with a weight of .+-.1, for example. The reverse, namely the use of a sigma-delta modulator as a digital/analog converter (D/A converter), is also possible. The digital input signal, for example, has a 16-bit word length with a scanning rate of, e.g., 48 kHz. The output signal can be, for example, a high frequency sequence of discrete analog values (for example .+-.1 volt with a scanning rate of 1 MHz). A sigma-delta modulator can also be used as a digital/digital converter (D/D converter), which converts a digital pulse sequence of high resolution (e.g. 16 it) and low scanning rate (e.g. 48 kHz) into a digital pulse sequence of low resolution (e.g. 1 bit) and high scanning rate (e.g. 1 Mz). An application for this type of D/D converter is described, e.g., in DE-A 198 19 069. This publication shows that an analog signal can be multiplied with a digital signal with the help of a purely digital sigma-delta modulator. This principle is used there for analysis of an analog sensor signal. Sigma-delta modulators of 2.sup.nd order with a one bit quantization are, for example, described in the article "A Use of Double Integration in Sigma Delta Modulation", by J. Candy, IEEE, Transactions on Communications", March 1985. The modulator described there comprises a 1.sup.st order modulator, which has an added feedback loop. It was pointed out in this reference that in the case of 1 bit quantization further feedback loops for increasing the modulation degree (degree .gtoreq.3) lead to unstable structures.
Topologies that guarantee the stability of a sigma-delta modulator with 1 bit quantization with suitable dimensions are described in "Theory and Practical Implementation of a Fifth-Order Sigma-Delta AID Converter", by R. W. Adams, et al, J. Audio Eng. Soc., Vol. 39, Nr. 718, 1991. The named article relates, for example, to an A/D converter with a 5.sup.th order sigma-delta modulator and a 1-bit quantization.
Sigma-delta modulators can also be used for other purposes than for A/D or D/A converter. For example, the use of several purely digital sigma-delta modulators inside an IIR filter was described in "IIR Filtering on Sigma-delta Modulated Signals", D. A. Johns, et al, Electronics Letters 14, February 1991, Vol. 27, Nr. 4. Non-linear operations on a data stream of a sigma-delta modulator is described in "Nonlinear Arithmetic Operations on the Delta Sigma Pulse Stream", M. Freeman, et al, Signal Processing 21, Elsevier Science Publishers, pp. 25 to 35 (1990).